The present invention relates generally to automated assembly systems more specifically the invention pertains to a system and a process which can be used to identify three dimensional coordinates and orientation to a robot.
The use of robots in manufacturing has been rapidly growing for over a decade. Commonly, robots are used for purposes of improving quality and increasing productivity. However, robots are being applied to situations where the robot design and the associated control systems are insufficiently advanced for satisfactory performance. Many robots in present use were first designed for simple operations such as pick and place or spot-welding where accuracy and kinematic performance were not crucial. However, newer robot applications, such as assembly, deburring, seam-welding, inspection, machining, drilling and the like require better performance and accuracy than that originally required of earlier robot systems.
Present methods of identifying three-dimension (3D) coordinates to a robot generally involve physically moving the robot and any attached end effector into the desired position, and then commanding the robot to "remember" the position for later reference. In many robotic systems, this procedure is impractical and undesirable for several reasons. First, it creates a safety hazard for the operator, as does any movement of the robot arm with an operator in close proximity. There is also a "hazard" for the workpiece and the robot itself. Second, in many production robotic systems, the throughput of the system would be greatly improved if the "teach" operations could be done "off-line"--that is, out of the work area of the robot. This is not possible if the robot arm itself is the teaching device. Third, manual movement and jogging of the robot arm is a time consuming and tedious process, and severely affects the throughput of the system. In addition to three-dimensional coordinates, it is commonly necessary to also specify orientation to the robot. A secondary "orientation" mode of operation allows for this.
Clearly a better method of identifying 3D locations to the robot system is required. This method should have the capability of being used in an off-line mode, and must not present a safety hazard to the operator, the robot, or the workpiece. It must also be easy and efficient to use from an operator standpoint.
One of the most difficult problems is that of determining the positional accuracy of an industrial robot throughout its large work zone. Just the fact that a robot has been instructed to assume some position does not guarantee that it has in fact done so. Several positional problems exist, and for this reason three-dimensional surface geometry acquisition is an important area of research for a wide variety of micro and macroscopic three-dimensional surface geometries. Several classes of problems are involved, such as long distance landscape measurement via radar and earth surface elevation via satellite laser altimeters. However the problem of accurately determining the surface geometry of a numerically machined part for example is not well-solved.
The task of providing a method identifying three-dimensional coordinates and orientation to a robot is alleviated to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,957,369 issued to Antonsson; PA1 U.S. Pat. No. 4,714,339 Issued to Lau et al; PA1 U.S. Pat. No. 4,590,578 issued to Barto, Jr. et al; PA1 U.S. Pat. No. 4,529,316 Issued to Di Matteo; PA1 U.S. Pat. No. 4,644,633 issued to Jones et al; PA1 U.S. Pat. No. 4,814,884 issued to Johnson et al; and PA1 U.S. Pat. No. 4,914,734 issued to Love et al.
The patents identified above relate to methods and apparatus for measuring or tracking surface coordinates. In particular, the Antonsson patent describes an apparatus and technique for measuring three-dimensional surface geometries. The apparatus comprises lateral photo-effect diodes and a light source capable of providing a tightly-focused, pulsed light spot. The light spot is focused on and scanned across the surface to be measured. The diode detectors are mounted in the focal plane of a pair of cameras, and return azimuth and elevation information for each spot. With knowledge of the location and orientation of the cameras, as well as calibration corrections of each camera, a computer can utilize the azimuth and elevation information to reconstruct the full three-dimensional location of each reflected light spot. The Lau et al patent is directed to a tracking system for measuring the spatial coordinates of a target. The system provides for a collimated beam to be directed to the target. A mirror attached to the target reflects the beam back to a tracking point, where photosensors provide error signals to a servo system which controls optics at the tracking point. The error signals are indicative of the direction necessary to accomplish the coincidence of the beams. An interferometer interferes the source beam with the beam that has travelled twice between the tracking and target points in order to measure separation. By measuring the directions of the beams relative to structure attached to the tracking and target points, the target point can be located in spatial coordinates.
The Barto, Jr. et al patent relates to a system for the off-line programming of a robot. The system provides for a workpiece to be mounted to a fixture and located within a robot's work envelope. Based on stored data, the robot touches-off on the fixture to determine the gross orientation and displacement of the workpiece. Based on that data, a coordinates transformation is determined which is applied to stored coordinates which define the locations on the workpiece for local features. The position of the local features on the workpiece is then measured to compensate for robot inaccuracies. Based on the data thus gathered, a second coordinate transformation is determined for correction of stored machine operation location coordinates.
The Di Matteo patent describes a method for eliminating erroneous data in three-dimensional optical sensors. According to the method, the surface location of points is measured with two different sensors, and false data reports are detected and eliminated by cross-correlating the data obtained from two different optical detector locations. To obtain the data reports, a point on a surface being investigated is illuminated, and the light directed onto a detector. The location of the point is computed by triangulation methods. The light applied to the surface at the point being investigated is also reflected in a different direction onto another detector, which uses the detected reflected light to compute the position of that point on the surface. The two computations are compared, and if the results are identical, then the point is designated as a true point on the surface.
The Jones et al patent describes the Westinghouse robotic lead forming system that can make use of the present invention.
The Johnson et al patent includes a three-dimensional pixel window generator which maps frames of serially scanned data.
The Love et al patent uses intensity area correlation to map terrain by passive infrared emissions.
While the above-cited references are instructive, the need for mapping three-dimensional coordinates for robotic assembly systems represents an ongoing technical interest. The present invention is intended to help satisfy that need.